The present invention relates to ion-implantation utilized for manufacturing a thin film transistor (TFT) of a liquid crystal display apparatus and an ion-implantation method.
To respond to a request that a drive circuit should be integrally formed on an array substrate in a step of manufacturing a liquid crystal display apparatus, studies and developments have been made to a poly-silicon thin film transistor (p-SiTFT) which is a thin film transistor (TFT) using a poly-silicon-based semiconductor layer.
Generally, in a poly-silicon thin film transistor, a low-resistance semiconductor layer used for source regions or drain regions is formed by ion-implantation at a high density. In a N-channel transistor, it is advantageous to adopt a lightly doped drain (LDD) structure in order to relax concentration of an electric field to the vicinity of a drain. A method of controlling the density of ions to be implanted has been widely used. In addition, to control the threshold voltage of the transistor, impurities of a much lower density than the LDD is doped into the channel region. In this case, an ion-implantation method is used.
In ion-implantation as described above, the amounts of ions to be implanted are as follows. With respect to a low-resistance layer for source and drain regions, phosphorus (P) or boron (B) is about 1.times.10.sup.14 to 1.times.10.sup.16 [cm.sup.-2 ]. With respect to LDD regions, P is about 5.times.10.sup.12 to 5.times.10.sup.13 [cm.sup.-2 ]. With respect to doping to channel regions, P or B is about 1.times.10.sup.11 to 3.times.10.sup.12 [cm.sup.-2 ].
Thus, even the dose amount of ion-implantation varies over a range of four to five digits depending on the portions where ion-implantation is performed. In many cases, ion-implantation apparatuses having different specifications specialized for a large current and a small current are used for manufacturing a mono-silicon transistor, for example.
However, there is a problem that the manufacturing efficiency cannot be improved if apparatuses specialized for respective steps are prepared, in response to a demand that a device such as a liquid crystal display apparatus or the like should be manufactured in manufacturing steps less than those in a mono-silicon device, like a liquid crystal display apparatus. From the viewpoint of investment productivity, a plurality of steps need to be completed by one same apparatus, and the dose amounts ranging by about four to five digits should desirably be subjected to ion-implantation by one same apparatus, in ion-implantation.
Actually, in doping into source and drain regions at a high density, a beam current density of 10 [.mu.A/cm.sup.2 ] is required to reduce the processing time per sheet of substrate to about 30 seconds. Meanwhile, to achieve doping at a low density which is applicable to channel regions, the current value of an ion beam must be reduced and the doping time must further be shortened. However, the lower limit of a controllable time as an effective doping time is about 0.2 second depending on the method of scanning the substrate. In order to control 1.times.10.sup.11 [cm.sup.-2 ], the current density of an ion beam must be controlled to be 80 [nA/cm.sup.2 ] or lower. Further, in case of a method in which the entire surface of a substrate is doped at once, the lower limit of a controllable time as a doping time is about 10 seconds and the current density of an ion beam must be controlled to be 1.6 [nA/cm.sup.2 ] or lower.
Normally, discharging power is changed in order to change the current density of an ion beam, for example, in case where high frequency discharging is utilized. The change of current which can be controlled by only changing the discharging power is about three digits. Further, the rate of ions generated in a plasma changes depending on the kind of impurities if the discharging power is changed. Particularly, in an ion doping apparatus having no mass separation function for manufacturing a liquid crystal display apparatus, changes of rates of ions of different impurities becomes a problem.
For example, various kinds of ions such as P.sup.+, PH.sub.x.sup.+, P.sub.2.sup.+, P.sub.2 H.sub.x.sup.+, P.sup.++, PH.sub.x.sup.++, H.sup.+, H.sub.2.sup.+, and H.sub.3.sup.+ are generated by high frequency discharging using a mixed gas of PH.sub.3 and H.sub.2. However, if the discharging power is increased, the rates of H.sup.+, H.sub.2.sup.+, and H.sub.3.sup.+ tend to be increased and the rate of dopant ions tends to decrease. On the contrary, if the discharging power is decreased, the rates of H.sup.+, H.sub.2.sup.+, and H.sub.3.sup.+ decrease and ion generation sources such as P.sub.2.sup.+ and P.sub.2 H.sub.x.sup.+ increase rather than P.sup.+ and PH.sub.x.sup.+. That is, if the discharging power is decreased to perform doping at a low density, there is a problem that implantation depth of P is small.
A method of diluting a gas may be considered as a method for decreasing the beam current of dopant ions. For example, if PH.sub.3 is diluted to 1% or 0.1% with use of H.sub.2, the rate of ion generation sources relating to P in a plasma can be reduced to 10% or lower, and thus, it is possible to respond to doping at a low density to some extent. However, this method has a problem that the rate of the ion generation sources changes in accordance with elapsed time or due to soil on chamber walls or the rate of undesired impurities such as carbon (C) and oxygen (O) mixed therein increases. Further, the change of the rate of ion generation sources directly influences the threshold voltage in channel doping, and in case of a LDD, the change of the density influences a characteristic of the transistor is turned on and the drain proof against voltage of a transistor, resulting in a serious problem. Also, C and O mixed deteriorate the performance of the transistor.
Another method will be a method of changing the aperture of an electrode portion from which ions are extracted. For example, this is a method of providing an electrode having a large aperture and a detachable movable electrode having a small aperture and of controlling the ion beam current by putting the movable electrode like a shutter. Where the aperture is set to 1/100, the current value is expected to be reduced by about two digits. However, in this method, if the aperture is changed, the pressure in the plasma chamber changes so that the plasma condition is changed, thereby changing the rate of ion generation sources. It is therefore impossible to lower only the ion beam current. In addition, in many cases, the uniformity of the ion beam current is generally deteriorated if the aperture is decreased. This method is therefore not suitable for practical use.
As has been described above, it is difficult to control the dose amounts with high accuracy over a wide range and to manufacture a field effect transistor at low costs by using a conventional method of controlling an ion beam current in ion-implantation.